Among conventional rewritable optical recording media in which information is recorded by means of laser light or other light is an magneto-optical recording medium, which has been put to practical use in some fields. In this system, information is recorded by creating regions of reverse magnetization in a recording material layer by means of light energy and magnetic field, and reproduced signals are obtained by detecting differences in the angle of Faraday rotation or Kerr rotation due to magnetization direction. This system, however, is disadvantageous in that since there is no practical method for rewriting information at least within one sector, it has been of limited application.
Another rewritable optical recording medium is a so-called phase-change optical recording medium, which utilizes the crystalline-amorphous phase change and is now being studied. In this system, information can be rewritten within one sector by means of two light beams (that is, recorded information is erased by the first beam and new information is then recorded by the second beam). Another advantage of this system is that in the case of a recording medium employing a recording material which shows a short crystallization time, overwriting (simultaneous erasing and writing) can be performed by means of one light beam, so that this system is of wide application in many fields.
As recording materials for use in such phase-change optical recording media, GeTe (Appl. Phys. Lett., 49 (1986) 502), In-Se-Tl-Co (Jpn. J. Appl. Phys., 26 Supplem. (1987) 67), and Ge-Sb-Te (JP-A-63-225934) have been proposed. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In general, recording on the phase-change optical recording medium is conducted by focusing a light beam, such as a semiconductor laser beam, on the layer of a recording material, such as those mentioned above, to melt the recording material in the irradiated regions and then rapidly cooling the melted recording material to form regions in an amorphous phase in accordance with the information to be recorded. For erasing the thus-recorded information, the amorphous-phase regions are irradiated with a focused light beam having a low power compared to the light beam used for recording to maintain the recording material in the irradiated regions at a crystallization temperature for a certain period of time, whereby the phase of the regions are changed back into crystalline one.
Recording materials for use in such phase-change optical recording media are preferably ones which show short crystallization times from the standpoint of simplifying the optical system or attaining improved phase change speed, and which are highly stable in their amorphous phase from the standpoint of storing recorded information for a prolonged period of time.
Among the above-mentioned conventional phase-change type recording materials, Ge-Sb-Te has been thought to be one of the best materials which possesses the above two properties. Illustratively stated, the Ge-Sb-Te disclosed in JP-A-63-225934 is prepared by mixing Sb.sub.2 Te.sub.3, which has a short crystallization time but is insufficient in amorphous phase stability, with GeTe, which is good in amorphous phase stability but has a long crystallization time, and hence the Ge-Sb-Te has been regarded as intermediate in properties between Sb.sub.2 Te.sub.3 and GeTe.
However, the GeTe part constituting the above Ge-Sb-Te disadvantageously has the property of not rapidly changing into amorphous phase, as described in Appl. Phys. Lett., 49 (1986) 502, and hence, the Ge-Sb-Te also has the defect of being less apt to form an amorphous phase as compared with Sb.sub.2 Te.sub.3. For this reason, optical recording media employing such Ge-Sb-Te have a disadvantage that cooling after melting should be conducted at a considerably high rate in order to attain stable recording.
Although the cooling rate may be heightened by increasing the moving speed (linear speed) of the light spot, this method is disadvantageous in that as a result of the increased linear speed, the time during which regions of information to be erased are maintained at a temperature not lower than the crystallization temperature in erasing (crystallization) operations is reduced, resulting in insufficient erasing. That is, in the case of the optical recording media employing the above-described Ge-Sb-Te, even if the recording material is improved so as to have a short crystallization time, it becomes necessary to increase the linear speed because such improvement results in difficulties in changing the recording material into amorphous state. Thus, satisfactory erasing properties cannot be obtained. The recording media of the above kind have had problems that the quality of reproduced signals is poor due to the information remaining unerased, waveform distortion at the edges, etc., and that the increased linear speed leads to decrease in recording sensitivity.
In place of increasing the linear speed, there may be a method in which the cooling rate is increased by providing the optical recording media with a cooling layer having a high thermal conductivity. This method, however, is also disadvantageous in that the increased cooling rate leads to decrease in erasing property and in recording sensitivity, as in the above-mentioned case, and that the production costs for the optical recording media are raised.
Further, the above-described Ge-Sb-Te optical recording media require high intensity light source such as semiconductor laser to enable stable recording by way of changing the phase from a crystalline state to an amorphous state. Furthermore, the S/N of the recording media varies to a great extent even with slight changes in output of the focused light beam. For the reasons, the recording media are subjected to exposure to high intensity light every time when rewriting the recorded information, and in turn, the recording material used therein is deteriorated by heat to cause changes in recording characteristics for a short period of time.